The concept of radiant gas burners is well known in the art. A radiant gas burner converts chemical energy within a combustible gas, usually a gas mixture of either air or oxygen (O.sub.2) and a combustible fuel, such as methane (CH.sub.4), into radiant energy, which is a form of electromagnetic radiation.
There are many types of radiant gas burners in use today, but most of them contain the following basic structural components: a gas inlet for receiving the fuel, a combustion chamber wherein the fuel is ignited, and a radiation element for emitting radiant energy based upon heat transferred thereto by the combustion process. The designs of such burners and the materials used in their construction vary considerably, but the main objective is invariably to heat the radiation element to the highest possible temperature via convective heat transfer from the combustion process, while at the same time inhibiting deformation, cracking or, other physical damage to the burner structure.
In the recent past, porous ceramic layers have been used for constructing radiation elements in radiant gas burners. Generally, porous ceramic layers can be heated to much higher temperatures than those temperatures attainable with metal radiation elements, such as metal grids, without degradation or deformation in structure. In these types of radiant gas burners, fuel is passed through the porous ceramic layer and fuel combustion occurs adjacent to and sometimes partially within a surface of the porous ceramic layer. In addition to achieving higher radiation intensities, a porous ceramic layer has a multiplicity of combustion zones situated therein near the burner surface, which result in a high combustion efficiency. The porous ceramic layers may be heated to temperatures well above 1400.degree. C. without significant degradation in structure. In fact, the bonded hollow sphere foam can be heated to at least 1700.degree. C. during operation without decomposition. Burners with a metal radiation element can withstand temperatures only up to about 1200.degree. C., due to oxidation of the burner structure.
The porous ceramic layers can be fabricated from any of a number of ceramic compositions, including mullite (3Al.sub.2 O.sub.3 .multidot.2SiO.sub.2), alumina (Al.sub.2 O.sub.3), zirconia (ZrO.sub.2), silicon carbide (SIC), and other materials. Moreover, the infrastructures of porous ceramic layers can vary. An example of one type of commercially available porous ceramic layer which can be used as a radiation element is "reticulated ceramic." This type of ceramic is characterized by numerous bonded struts and is described in detail in, for instance, U.S. Pat. Nos. 4,608,012 to Cooper and 3,912,443 to Ravault et al. Another example of a porous ceramic layer suitable for use as a radiation element is "bonded hollow sphere foam", or "hollow microsphere foam." This type of ceramic is characterized by a network of hollow ceramic spheres which are bonded together and the spheres are described in detail in, for instance, U.S. Pat. No. 4,671,909 to Torobin. Bonded hollow sphere foam is commercially available from and manufactured by Ceramic Fillers, Inc., Atlanta, Ga., U.S.A., and is sold under the trademark "Aerospheres.TM.."
Although the use of porous ceramic layers as radiation elements in radiant gas burners has increased the intensity and efficiency at which chemical energy in fuel can be converted into radiant energy, the designs of radiant gas burners using porous ceramic layers remain in a state of infancy and their efficiencies are less than optimal. Accordingly, a need exists in the industry for new and improved radiant gas burners utilizing porous ceramic layers which exhibit higher efficiencies and higher radiation intensities than presently known burner designs.